Federal agents are trying to determine how a suspected Ohio white supremacist with a felony conviction for manslaughter acquired a cache of 18 assault weapons and other firearms, along with high-capacity magazines and more than 40,000 rounds of ammunition, according to federal law enforcement officials and court documents reviewed by NBC News.

The storehouse of weapons was discovered late last month when FBI agents arrested Richard Schmidt, 47, the owner of a Bowling Green sporting goods store called Spindletop Sports Zone, on charges of marketing counterfeit goods -- such as football jerseys with NFL logos -- from China.

Although initially portrayed as a probe into the thriving international market for counterfeit clothing, the case took a surprising turn this week when the U.S. Attorney’s Office in Cleveland unsealed search warrants and an indictment also charging Schmidt with illegal possession of firearms.

According to the documents, FBI agents who searched Schmidt’s sporting goods store and four trailers behind it, found a stash of weapons that included AR-15 assault rifles, Ruger and Sig Sauer semi-automatic pistols, bulletproof body armor and high-capacity magazines as well as ammunition.

The agents also discovered evidence of Schmidt’s ties to the neo-Nazi movement, documents show. Among the evidence seized, according to search warrants, was a video of a national convention of the neo-Nazi National Socialist Movement; bumper stickers of the National Alliance party, another neo-Nazi group; a “Jewish 500” list -- a supposed roster of Jewish-owned businesses -- and paraphernalia from the “Waffen SS,” Adolph Hitler’s Nazi military force in Germany from the early 1930s through World War II, according to the search warrants.

A federal law enforcement official, who spoke with NBC News on condition of anonymity, said that FBI counterterrorism agents involved in the case had picked up evidence that Schmidt may have been planning attacks against Jewish and civil rights groups in the Detroit area. “This is an active investigation,” said another federal law enforcement official when asked if Schmidt was believed to have been working with any others in the neo-Nazi movement.

In the indictment unsealed this week, Schmidt was charged with three counts of illegal possession of firearms, ammunition and body armor and one count of trafficking in counterfeit goods.

Schmidt’s lawyer, federal public defender Andy Hart, did not respond to a request for comment.

The law enforcement officials said the case appears to illustrate some of the gaps in current background checks for gun purchasers that President Barack Obama has proposed closing as part of his package of executive actions and legislative proposals released this week aimed at curbing gun violence. Schmidt was charged with murder and felonious assault in 1989 after killing a Hispanic man and shooting two others with a semi-automatic pistol during a traffic dispute. He later pleaded guilty to voluntary manslaughter and was sentenced to 10 to 25 years in prison. Federal officials were not immediately able to provide information on when he was released from prison.

Despite a federal law that prohibits convicted felons from buying firearms, Schmidt was still able to acquire his stockpile – though authorities don’t yet know how he acquired them. Federal agents have been trying for weeks to trace the weapons, but officials said they have so far made little progress. This could indicate that Schmidt purchased his weapons from private dealers or gun shows, where background checks are currently not required, one official said. But he also could have obtained them on the black market.

“It is deeply troubling that law enforcement found this man, with a prior homicide conviction, in possession of an arsenal,” said Steven M. Dettelbach, the U.S. attorney for Cleveland.

Mark Potok, who tracks hate groups for the Southern Poverty Law Center, said the group had found an entry that appeared to be from Schmidt on a neo-Nazi website several years ago, using the Yahoo profile of “Vinlander 101” and declaring his plans to set up a “historical preservation” group. (One of the trailers behind Schmidt’s sporting goods store was registered to the “Vinland Preservation League” -- a now defunct nonprofit.) He noted that the use of the word “Vinland” was likely inspired by the “Vinland Social Club,” a now largely dormant neo-Nazi skinhead group that emphasized the early Vikings role in colonizing the American continent.

“The sad reality is there are people around this country who are building up enormous arsenals of weapons because they think the end is coming -- either a race war, or the new world order … or some other form of apocalypse,” he said.